In a medium saucepan, whisk together the blood orange juice & zest, granulated sugar, egg yolks, egg, and salt until combined. Place the saucepan over a medium low heat. Whisk continuously. The curd will get a bit foamy, as it cooks and thickens, the foam will begin to turn into larger bubbles and the curd will start to look smooth. You want the curd to be thick enough to coat the back of a spoon. This should take about 10 minutes. Once the curd has thickened, remove from the heat and stir in the butter 1 Tbsp at a time. Strain the curd into a clean bowl. Cover with plastic wrap so that the wrap is touching the surface of the curd and refrigerate until cooled and thick.